I'm running new electric in a friends basement and so far uncovered a
lot of bad wiring jobs, buried splice boxes, you name it. So now I
noticed that the central A/C compressor is wired with 10 guage wire.
I'm pretty sure its suppose to be 8 guage. And also the existing 10
guage wire is hooked up to a 40 A breaker, which of course is not
right, its suppose to be a 30A breaker.
So since everything is wide open now, I was wondering should I run
new 8 guage wire to the A/C? The unit is a Lennox, I beleive a 3 ton
unit, and the plate says "Min circuit ampacity 24.4 amps" and it also
says " Max fuse or ckt bkr 40A".
Even if the unit has been running fine all these years, I'm wondering
what happens down the road if it needs to be replaced with a new unit
that needs more amperage. the existing 10 guage wire will not be
sufficient I'm thinking.
OK hold on to your hat.
That label says max fuse or breaker 40a ... that is what you have
It also says minimum circuit ampacity 24.4a so the wire can be 12 ga.
You are larger than you have to be with 10ga.
That is what happens in article 440. They know you need a big breaker
to get the compressor going but the actually running current is much
lower. If you look at 310.15 you will see #12 is actually rated at
The common axiom that 14ga = 15a, 12ga = 20a comes from 240(4)(D)
where they build the 80% safety factor in. (it is actually the max
breaker size) It does not apply on dedicated motor circuits
Homeowners will keep plugging things in until the breaker trips, then
unplug the clock.
That label has it built in already by the engineer. You will see that
compressor actually runs more like 16-18a on a hot day.
The rules are different for motors than on other things for what
it's worth. Wire is supposed to be sized at 125% of the full load
amperage shown on the appropriate chart. Overcurrent protection can be
175% of the value shown in the chart. One can go to the next larger
size if the 175% calculation puts one in between standard fuse or
This might not apply directly to air conditioners but I think the
code writer's reasoning is the same.
Nope You use the label to size the circuit and the breaker. An
engineer designed it and an inspector should sign off on it.
The 10 gauge, or even 12 gauge wire is adequately protected from a
short circuit by the 40a breaker. Overload protection is provided
inside the compressor.
That is the way it works on dedicated motor circuits.
In fact there is a question on most inspector tests and the right
answer is 14 gauge wire, 40a breaker (1 HP motor @ 120v)
normally a new install like a replacement AC will get all new wring
and service disvconnect. a new 8 gauge copper line with new disconnect
by the compressor will see less voltage drop, a good thing.
but new compressors will likely be more efficent and run with less
if everything is open then this is the best time to upgrade, and if
breaker space is available run a couple 20 amp lines to work boxes for
future use if needed. I did that once and a few years later was glad I
did:) when we decided to install a new kitchen
In The U.S., standard gauge copper wire for a 20 amp circuit is 12 gauge
and 10 gauge for a 30 amp circuit. Below is a link to a chart
for standard current ratings for home wiring. There are all sorts
of different ratings for different insulation types, wire in free air,
in conduit or buried but the chart shows what any electrician would
go by if wiring a home. Aluminum wiring is usually one gauge size larger
for the same current carrying capacity as copper. ^_^
I had a 3.5 ton installed. They used 10 gauge and 30 amp breaker. After
reading data plate and measuring operating current I switched to 20 amp
It's only drawing 6.5 Amps and no problems. Never hurts to use thicker
The circuit breaker is meant to protect the wiring not necessarily the
equipment which will normally have its own secondary protective devices.
The 20 amp breaker won't harm anything but if there is a surge current
greater than 20 amps from the AC unit starting on a hot day, you may get
nuisance tripping of the breaker. O_o
On Mar 3, 11:16 pm, The Daring Dufas <the-daring-du...@stinky-
A 20 amp breaker is less than the rated 24 amp min
circuit capacity on the eqpt label. As others have stated, the
existing 12 gauge wiring and 40 amp breaker are
correct, meet code and no change is required.
On 3/4/2013 6:13 AM, firstname.lastname@example.org wrote:
Not around here. Every electrical inspector I know would reject a 12
gauge wired circuit with a 40 amp circuit breaker. A 40 amp circuit
uses #8 copper NM/UF 60°C cable or #8 aluminum SE/USE 75°C cable. An
electrical inspector may require #6 aluminum be used depending on the
jurisdiction's requirements which may be stricter than The NEC. This
applies to "homes" not necessarily industry which will normally use
different wiring methods and higher temperature insulated wiring. I've
wired homes, businesses and industry for a living so I know a little
about electrical wiring. I've also worked in commercial sales of
electrical products. The circuit breaker is meant to protect the wiring
and should always be sized accordingly. There are places that have no
electrical inspection department and I suppose you can do whatever you
want. I've worked in those areas too and seen extremely dangerous wiring
that was quite scary. ^_^
On Mar 4, 7:50 am, The Daring Dufas <the-daring-du...@stinky-
Then they are not following the NEC and either they don't know
what they are doing or that particualr AHJ has unusual requirements.
A 40 amp circuit
You're trying to apply the NEC rules for general purpose branch
circuits to motor eqpt that is on a dedicated circuit with
it's own over-current protection. As the Mike stated, the eqpt
label says the minimum circuit ampacity is 24 Amps. That translates
into 10 guage wire being fine. Actually, 12 would also meet it.
It further states that a 40 amp breaker is the max allowed, which
is what he has. Presumably any competent electrical inspector
would read the eqpt label and follow it.
Theoretically any AHJ could make up anything they want. But #6
for this 3 ton AC is nuts.
Then you should know that what Mike has is perfectly fine.
It exceeds the eqpt label, which is what governs here and it
You're ignoring the fact that the eqpt has it's own over-current
What Mike has is not dangerous. Itfully meets or exceeds NEC in
On 3/4/2013 7:56 AM, email@example.com wrote:
I'm not going to argue with you over it except to tell you it would not
pass inspection in my area. I don't argue with inspectors either because
they can call people who carry guns and handcuffs. ^_^
On Mar 4, 10:50 am, The Daring Dufas <the-daring-du...@stinky-
Then maybe you can tell us what rules the inspectors
are following and where your area is, because it's entirely
consistent with NEC. And it would pass inspection here,
in NJ. Or do your inspectors just make stuff up as they
On 3/4/2013 10:27 AM, firstname.lastname@example.org wrote:
Everywhere I've worked followed the NEC even on U.S. installations
overseas where The Army Corps of Engineers took care of inspections.
All I know is from my own experience working on various jobs over
four decades. Perhaps you have much more experience in the field of
electrical work than I do but I can only refer to my own experience.
Perhaps you are an experienced electrician in your city/state and know
how things are done in your kingdom but from your post, it appears you
follow different standards. I must contact my relatives in New Jersey
and implore them to run, run like the wind! ^_^
On Mar 4, 11:50 am, The Daring Dufas <the-daring-du...@stinky-
Then you should be able to cite for us the section of NEC
that says the installation that Mikepier has is in violation.
I'm an electrical engineer. And gfretw has weighed in on
the issue. I believe he's an electrician. And he said the
same things I did, in particular that what Mike has is code
compliant and does not need to be changed.
We follow the NEC here. So show us where the NEC says
we're wrong..... And until them, stop spreading FUD.
There is absolutely nothing unsafe, dangerous or in violation of NEC
what Mike has. This is a common confusion. Your mistake
is applying the rules for branch circuits for lights, receptacles, etc
to HVAC eqpt. Different rules apply for obvious reasons and if you'd
just take the time to read the NEC, you'll see that.
On 3/4/2013 11:47 AM, email@example.com wrote:
How often do you deal with electrical and mechanical inspection
services? I've done electrical and HVAC work for a living and I
base my assertions on practical experience where I've worked. It
may be different where you did your electrical and HVAC work for
a living. Perhaps the authorities interpret the NEC differently
where you have done your professional electrical and HVAC work?
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